1. Field of the Invention
The present invention relates generally to a mobile communication system and more particularly, to a Channel Quality Indicator (CQI) reporting method for a Dual-Cell High Speed Downlink Packet Access (HSDPA) service of a mobile communication system that is capable of reducing CQI reporting overhead.
2. Description of the Related Art
Universal Mobile Telecommunications System (UMTS), which is one of the Third Generation (3G) mobile telecommunication technologies based on the Wideband Code Division Multiple Access (WCDMA), has evolved from Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS), and provides mobile and computer users with packet-based multimedia services including text messaging and voice and video communication services in a consistent service environment.
The UMTS system supports HSDPA service in order to enhance packet data transfer speed and capacity in the downlink.
In order to secure a high-speed data transfer rate, HSDPA uses Adaptive Modulation and Coding (AMC) and Hybrid Automatic Repeat reQuest (HARQ). The Quadrature Phase-Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (16QAM), and 64QAM modulation schemes are selectively used with the AMC. The AMC technique selects the modulation scheme, coding format and coding rate based on a channel condition and channel type between the base station and the User Equipment (UE) so as to improve throughput of the entire cell.
HARQ is implemented with fast retransmission and soft combining techniques. When an erroneous packet is detected, the retransmission is triggered between the base station and the UE. The recipient part combines the retransmitted packet with the previously received packets, thereby reducing the number of retransmission requests and improving overall efficiency. In order to support the HSDPA service, the base station and the UE exchange control information such as Orthogonal Variable Spreading Factor (OVSF) codes, number of OVSF codes, Transport Block Size (TBS), Modulation and Coding Scheme (MCS), channel index information for HARQ, CQI for indicating the channel condition, and HARQ ACK/NACK.
FIG. 1 is a sequence diagram illustrating operations of HSDPA system comprising a cell and a UE.
Referring to FIG. 1, a UE 102 first transmits a CQI to a cell 101. Since the UE 102 does not know when data is transmitted in the downlink, it transmits the CQI information periodically in step 103. When there is data to be sent, cell (or Node B) 101 performs scheduling based on the CQI. In the scheduling process, Node B determines a number of code channels available for allocation and an MCS level. Such information is transmitted to the UE 102 through a High Speed-Shared Control Channel (HS-SCCH) in step 105. The HS-SCCH is received by the UE 102 in a Transmission Time Interval (TTI), and the UE 102 receives data by demodulating the High Speed-Physical Downlink Shared Channel (HS-PDSCH) in step 106 with reference to the HS-SCCH. In order to make a status report for HARQ, the UE 102 performs a Cyclic Redundancy Check (CRC) to determine Acknowledgement/Non-Acknowledgement (ACK/NACK) in step 106-1. If the data is received in error, the UE 102 transmits a NACK to Node B 101 to request retransmission of the data; and otherwise, transmits an ACK to Node B 101 in step 107. The status reports of ACK/NACK of step 107 and periodic CQI of step 108 are transmitted through the High Speed-Dedicated Physical Control Channel (HS-DPCCH).
FIG. 2 is a timing diagram illustrating transmissions of the physical channels of an HSDPA system. As shown in FIG. 2, CQIs 205, 206, and 207 are periodically transmitted via the HS-DPCCH. Node B transmits two slots of HS-SCCH 201 before it begins transmitting HS-PDSCH 202 in order for the UE to check the information on the demodulation of the HS-PDSCH. ACK/NACK information 204 is transmitted 7.5 slots 203 after the transmission of the HS-PDSCH 202 in consideration of the demodulation and decoding of the data carried by the HS-PDSCH 202.
Dual-Cell HSDPA operation is described hereinafter with reference to FIG. 3. Unlike the conventional HSDPA in which the UE measures received signal strengths of the cells and connects to the most appropriate cell based on the measurements, the Dual-Cell HSDPA is characterized in that a UE 308 connects to two different cells 301 and 302 defined by two different carriers 303 and 304 of a Node B. The UE 308 receives the HSDPA signals 306 from the first cell 301 in the first carrier f1 304 and HSDPA signals 307 from the second cell 302 in the second carrier f2 303, simultaneously. In the WCDMA system, the transmission bandwidth of a cell is 5 MHz such that the UE must have a reception capability of 10 MHz for supporting Dual-Cell HSDPA. Since the HSDPA signals are received from two cells, the maximum transmission rate is doubled. In case of uplink, however, the Dual-Cell transmission function is not supported, whereby the uplink channel 308 is transmitted to only one cell. Even in downlink transmission, common and dedicated channels that are not related to the HSDPA are received from a single cell. Typically, the cell that is in charge of controlling the uplink channel and common and dedicated downlink channels is referred to as an “anchor cell” and the other cell is referred to as a “supplementary cell”. Although a description is provided with two cells (two carriers), the Dual-Cell HSDPA system can be implemented with multiple supplementary cells and an anchor cell. In order for the Dual-Cell HSDPA service to support the HARQ and AMC, the ACK/NACK and CQI should be transmitted to the respective cells, whereby the uplink channel permitted for the anchor cell must be configured to carry the ACK/NACKs and CQIs destined for the anchor and supplementary cells. A simple approach to achieve this includes code multiplexing in which two codes are assigned to the uplink for the anchor cell. This approach is simple but is problematic in that the increment of a number of channels to be transmitted increases a Peak to Average Power Ratio (PAPR) ratio, resulting in a reduction of uplink coverage.